What is the difference between running ./file.py and python file.py?

Question

When I run Python script from the command line

./file.py

it is interpreted differently(fails with a bunch of errors) from when I run it using:

python file.py

Why are they executed differently?

Answer 1

On Unix-like systems:

• ./file.py requires file.py to be executable (e.g., chmod a+x file.py).
• ./file.py runs the script with whichever interpreter is specified in its shebang line; python file.py runs it with whichever interpreter named python is highest on your $PATH. If you have multiple versions of Python, this can make a big difference.

If you don't know which python is highest on your $PATH, type which python and it will tell you.

If you want the shebang line to run the python that's highest on your $PATH, write it as:

#!/usr/bin/env python

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On Windows:

• ./file.py runs the script with whatever application is registered to handle *.py files, whereas python file.py runs it with whichever interpreter named python.exe is highest on your %PATH%. Again, if you have multiple versions of Python, this can make a big difference.

Note that Windows cmd.exe, unlike Unix shells, doesn't care about shebangs, only extensions. However, if you've installed a new enough version of Python, the application registered to handle *.py files will be the PEP 397 Python launcher, which does look at shebangs. (You can also get the launcher separately for older versions of Python.)

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From your comments, the first line is:

just a start of the program, i.e. import socket

It sounds like you don't have a shebang line at all. That means that, if you're on a Unix-like system, ./file.py will run it with the default interpreter—which is /bin/sh on Unix and most Unix-likes, which isn't a Python interpreter at all. So you will probably get a bunch of confusing errors, probably starting with something like import: command not found.